This invention relates to novel bulk electronically conducting glasses or "semi-conducting" oxide glasses. More particularly, the invention relates to novel glasses with bulk electronic conduction suitable for making microchannel electron multipliers, conducting faceplates, printer fibers, intensifiers, detectors, and electro-optical devices.
The common oxide glasses are mostly dielectrics. Under high electric fields they may support low levels of conduction based on electrolysis with ion mass transfer. This class of glasses is not suitable for use in microchannel plates unless it receives a surface heat treatment with hydrogen. This process step converts a thin surface layer to provide electron conduction, which is necessary for device application. For example, a glass containing lead oxide can be reduced in hydrogen to give a thin surface layer of electronically conducting lead. In such glasses, the bulk resistivity of the glass substrate remains high and the conduction mechanism remains ionic. The surface layer, however, is in an activated state, and becomes unstable under electron bombardment. Many practical problems arise from this material behavior.
A "bulk conducting glass" (hereinafter referred to as "BC glass") may be generally characterized in that (a) electrical conduction occurs predominantly by electrons (and/or holes) rather than by ions, and (b) the temperature coefficient of resistivity is negative.
Certain oxide glasses containing transition metal ions, such as V.sup.+5 /V.sup.+4 and Fe.sup.+2 /Fe.sup.+3, have electronic conduction. The conduction mechanism is the hopping process as explained by the Mott theory. Unlike covalent semiconductors such as doped Si and Ge, these oxide glasses (BC glass) have low conductivity. However, BC glass has a higher conductivity than common oxide glasses, and higher secondary electron emission coefficients than covalent semiconductors. They are believed to be good candidate materials for electron multiplication applications in such devices as gamma-ray ion detectors and night-vision imagers.
Baynton et al first reported that a family of V.sub.2 O.sub.5 P.sub.2 O.sub.5 glasses was bulk electronically conducting rather than ionically conducting (J. Electrochem. Soc., Vol. 104, p. 237, 1957). The first widely studied systems were those containing V.sub.2 O.sub.5 and Fe.sub.3 O.sub.4, their conductivity was considered as "hopping" of electrons or vacancies between transition metal ions of different valance V.sup.+5 /V.sup.+4 and Fe.sup.+2 /Fe.sup.+3.
Novel glass compositions (i) with bulk electronic conduction (ii) suitable for making conducting faceplates, printer fibers, multichannel electron multipliers and other electro-optical devices are highly desirable for several reasons. For example, printer fibers of BC glass can be made much easier than those of metal wire with coatings. The diameter of glass fiber can be drawn down to several microns, so the glass fiber is more flexible and the print has better resolution. Also, existing multichannel electron multipliers are made of surface-conducting glass, which can suffer from local overheating at the surface level. If multipliers were made of bulk-conducting glass, local overheating at the surface layer of the glass and ion feed back due to electric field would be reduced. Channel multipliers made of BC glass would also have higher electron gain, wider dynamic signal range, and longer service life.
Workability is an important factor to the formation of fibers for use in applications such as microchannel plates. The fibers used in microchannel plates range from 6-100 .mu.m and have length to diameter ratios (.alpha.) of between about 40 to about 100. Unfortunately, BC glass compositions based on vanadium-phosphate have not been suitably developed for use in commercial applications.
For example, U.S. Pat. No. 3,520,831 and U.S. Pat. No. 3,910,796 describe glass compositions having specific amounts of V.sub.2 O.sub.5 and P.sub.2 O.sub.5, as well as other specific ingredients. Notably, the '831 patent relates to surface coatings based on vanadium and phosphate compositions: the patent does not teach or suggest bulk conducting glass compositions that can be drawn into fine structures such as a fiber suitable for a microchannel plate. The '796 patent discloses specific BC-glass compositions based on vanadium-phosphate, but the patent also expressly directs that lead oxide, PbO, in the compositions, should not exceed 15 Mole % or the glass will have an inadequately high resistance to current. Unfortunately, contrary to the teachings of the present invention, this restriction on the amount of PbO is not necessary, and is a drawback that limits how workable the glass is. The use of PbO in excess of 15% is in fact necessary to the formation of a workable glass, and as taught herein, can be formulated into vanadium-phosphate glass compositions in excess of 15% without creating inadequately high resistance to current.
Accordingly, the prior art has not provided BC glass compositions that have good electrical properties, and that are workable into fibers, and further into microchannel plates, and other structures.